A particulate water absorbing agent containing a water absorbent resin as a main component is used in sanitary materials such as a disposable diaper, a sanitary napkin, and an incontinence pad. The water absorbent resin absorbs body fluids such as urine and blood. Known examples of the water absorbent resin encompass: a partially neutralized cross-linked polymer of polyacrylic acid; a hydrolyzed starch-acrylic acid graft polymer; a saponified vinyl acetate-acrylic ester copolymer; a hydrolyzed or cross-linked acrylonitrile copolymer; a hydrolyzed or cross-linked acrylamide copolymer; and a cross-linked cationic monomer. Among these, a partially neutralized cross-linked polymer of polyacrylic acid is particularly preferable in terms of performance and cost.
A polyacrylic acid (salt) water absorbent resin, particularly a particulate water absorbing agent containing a partially neutralized polymer of polyacrylic acid, is produced by the following method: First, a monomer containing acrylic acid salt is polymerized with use of a polymerization initiator so as to obtain a hydrogel. This hydrogel is dried and pulverized so as to obtain water absorbent resin particles. The water absorbent resin particles are then surface-cross-linked according to need with use of a surface cross-linking agent. Further, a surface reforming additive is added to surfaces of the respective water absorbent resin particles according to need so as to finally obtain a particulate water absorbing agent.
In order to obtain a high-quality particulate water absorbing agent, it has conventionally been necessary, for example, to sufficiently mix a monomer aqueous solution and the polymerization initiator during the polymerizing step. This requires the polymerization initiator to be in a form of a low-concentration aqueous solution. As a result, a large amount of such an aqueous solution is necessary, and also a tank for storing the aqueous solution needs to have a large capacity.
In a case where an operation of a production plant is suspended for a reason such as trouble in or maintenance of the production plant, a supply of the aqueous solution and the like for the polymerization initiator is also suspended. In this case, the aqueous solution is kept stored in a tank for an extended period of time. While the aqueous solution is stored in the tank, a chemical reaction may occur in the aqueous solution and unfortunately impair a function of the polymerization initiator. In view of the fact that such an aqueous solution having an impaired function is discarded, if the tank has a large capacity, an amount of the aqueous solution to be discarded is large accordingly. This not only is uneconomical, but also requires a long time and high cost for waste liquid disposal.
Another example method for producing a particulate water absorbing agent containing a water absorbent resin as a main component is a method of mixing an additive and the like by line mixing. This method, however, produces solutions which differ from one another in concentration and which are thus unstable in concentration. It follows that particulate water absorbing agents obtained in accordance with the method differ from one another in quality and are thus unstable in quality.
In relation to the above problems, the following techniques have been known conventionally: European Patent No. 0,873,185 discloses a particulate polymer obtained from a reaction between (i) a monomer and (ii) a polymerization initiator stored in a tank in which a temperature is controlled at a predetermined temperature.
U.S. Pat. No. 7,265,190 discloses a method for producing a water absorbent resin in which method a polymerization initiator is added to a monomer liquid while the monomer liquid is in a tube for transporting it. This production method allows property values of a water absorbent resin to be stable by removing a water absorbent resin which has a property value that falls outside a predetermined range. This method, however, requires mixing such a removed water absorbent resin again and thus requires a complicated process.
U.S. Pat. No. 4,985,518 discloses a method for producing a water absorbent resin in which method a polymerization initiator and monomer liquid stored in respective tanks are mixed in a line mixer and then fed to a belt polymerization device. This production method uses polymerization heat to simultaneously carry out polymerization and drying so as to obtain a solid water absorbent resin. The water absorbent resin thus obtained, however, has a low water absorbing property.
PCT international publications 2007/028746 pamphlet, 2007/028747 pamphlet, and 2007/028751 pamphlet each disclose a method for producing a water absorbent resin which method includes a continuous neutralization step in which an acrylic acid and a base are mixed with each other. In these methods, a liquid containing materials (i.e., a monomer and a polymerization initiator) of a water absorbent resin is stored in a tank.
PCT international publication 2007/023097 pamphlet discloses a method for producing a water absorbent resin which method includes two systems for a polymerizing step of a water absorbent resin but has a single system for a production process after the polymerizing step. With this production method, a water absorbent resin obtained has stable property values. This production method, however, requires a plurality of polymerization devices and thus requires a complicated process.
Methods similar to the above are also disclosed in U.S. Pat. Nos. 6,716,894, 6,817,557, 7,193,006, 6,727,345, and PCT international publication 2007/023097 pamphlet.